The invention is based on a measuring element as defined hereinafter for a device for determining the mass of a flowing medium, in other words its flow rate for the aspirated air of internal combustion engines.
In such devices, also known as air flow rate meters, the resistor tracks of a temperature sensor and a compensation resistor on the one hand and the resistor track of the measuring resistor on the other, together with two balancing resistors, form a Wheatstone bridge, whose bridge diagonal voltage is applied to a control amplifier. The output voltage of the control amplifier serves as a heating voltage for a heating resistor of the measuring element.
In a measuring element of this generic type for the aforementioned device, known from DE 36 38 138 A1; U.S. Pat. No. 4,777,820, the distribution of the resistor tracks on a substrate is made such that the resistor tracks succeed one another in the flow direction and are parallel. The resistor track for the compensation resistor is disposed between the resistor track for the temperature sensor and the resistor track for the measuring resistor, on the same side of the substrate, and the resistor track for the heating resistor is disposed on the opposite side of the substrate, directly facing the resistor track for the measuring resistor. The various resistor tracks are separated from one another by slits in the substrate that extend transversely to the flow direction, thereby bringing about a temperature decoupling between the resistor tracks. As a result of these separating slits, the substrate has three prongs of equal length, of which the first two prongs in the flow direction each carry one resistor track, and the last prong in the flow direction carries the resistor track for the measuring resistor and the heating resistor.
Because of their notch action, the ends of the various separating slits located inside the substrate decrease the strength of the substrate at the base of the notch. Particularly in the region of the middle prong, whose clamping cross section on both sides is defined by one slit end each, the strength is severely reduced, especially under dynamic load. As a result of the notches, a force flux that is uniformly distributed without notch action is interfered with, in other words compressed or deflected. This creates tension concentrations at the base of the notch. Tension peaks develop in these regions that are markedly higher than the rated tension.
During the assembly and upon impacts transmitted by structure-borne sound, the various prongs of the measuring element are made to vibrate; especially when the natural frequency is reached, and in combination with the notch action at the base of the notch of the separating slits, the vibration causes considerable tension peaks in the region at the ends of the slits of the various prongs of the measuring element. If these tension peaks exceed the characteristic strength values for the material, they lead to cracks beginning at the end of the slit and may even lead to breakage of the substrate.